In harmony with the downsizing of electronic equipment, a rapid progress is recently made toward higher integration of LSIs and toward ASIC (application specific integrated circuits). For LSI mounting, multi-pin thin-film packaging is widely employed. Such multi-pin structures include protruding electrodes or solder bumps of at least 10 μm in height as the connecting terminal, while the technique for forming solder bumps is requisite. When bumps are formed on LSI by a plating technique, a photoresist material is used. While bumps of mushroom shape are formed using conventional thin film resist, such bump shape is difficult to increase the integration density by increasing the number of pins on LSI or reducing the pin spacing. It is then necessary to shape bumps with vertical sidewalls (or straight sidewalls) utilizing a thick film resist. The thick film photoresist subject to plating must fulfill many features including high sensitivity, vertical profile, and high resolution, as well as deformation resistance and crack resistance of the pattern during or after the plating step.
As the means for solving these problems, certain compositions are known in the art. For example, JP-A H10-207057 discloses a positive photoresist composition having an acrylic resin added for the main purpose of improving the plating resistance of bump-processing resist film. JP-B S46-16049 discloses an alkali-soluble photosensitive resin composition comprising a novolac resin, a naphthoquinonediazide-containing compound, and polyvinyl ether. Both the compositions are improved in crack resistance, but have a possibility that the pattern profile is degraded owing to the reduced contrast of resist film. Also a positive photoresist composition comprising a novolac resin and a hydrolyzate of an alkyl vinyl ether-maleic anhydride copolymer having a molecular weight of 20,000 to 80,000 is known from JP-A H06-202332. This system, however, is insufficient with respect to crack resistance during or after the plating step in the gold plating application.
On the other hand, since solder bumps must have a height of several tens of microns to about 100 microns (μm), the resist pattern formed therefor must accordingly have a depth of several tens of μm to about 100 μm. It is thus recommended that the resist material be coated as a very thick film having a thickness of several tens of μm to about 100 μm. This implies that the resist material adapted for solder bump pattern formation may have problems with respect to sensitivity and resist pattern profile. While positive resist compositions comprising a novolac resin and a naphthoquinonediazide-containing compound are commonly used in the art, as described in JP-B S46-16049 and JP-A 2004-198915, thick films thereof having a thickness of several tens of μm to about 100 μm are degraded in sensitivity, which reduces the productivity efficiency of pattern formation, causes the pattern profile to be tapered, and leads to profile deficiency against the requirement to shape bumps with vertical sidewalls (or straight sidewalls). For this reason, the solder bump-forming resist material requiring a film thickness of several tens of μm to about 100 μm prefers chemically amplified resist compositions to the positive resist compositions comprising a novolac resin and a naphthoquinonediazide-containing compound because a pattern with more vertical sidewalls can be formed at a higher sensitivity.
In connection with the chemically amplified resist composition used as the solder bump-forming resist material requiring a film thickness of several tens of μm to about 100 μm, when polyhydroxystyrene in which some phenolic hydroxyl groups are substituted with acetal groups as acid labile group is used as the base resin (JP-A 2002-006503), a problem arises that a long time is necessary for development because of the thick film. The long development time leads to drawbacks such as a time-consuming process and low production efficiency.
With respect to the electrolytic plating bath used in solder bump formation, strong acid based solder plating baths, known as high-speed plating baths, are often utilized for enhancing production efficiency. When the pattern of chemically amplified positive resist composition is immersed in the strong acid based solder plating bath, the strong acid system exposes the pattern to very rigorous conditions, giving rise to the problems that not only cracks generate as mentioned previously, but also the pattern swells and deforms. The base resin commonly used in chemically amplified positive resist compositions is polyhydroxystyrene in which some phenolic hydroxyl groups are substituted with acetal groups as acid labile group (JP-A 2002-006503). When a pattern of the chemically amplified positive resist composition having such acid labile group is immersed in a strong acid based solder plating bath, the acid labile group can be eliminated by reaction with the acid in the plating bath. Thus the pattern is locally made hydrophilic and progressively swollen. As a result, the pattern is deformed within a short time.
Although the chemically amplified positive resist composition is preferred as the solder bump-forming resist material, many problems remain unsolved including a long development time, and film swell and pattern deformation upon immersion in plating bath.
Another exemplary chemically amplified positive resist composition used as the solder bump-forming resist material is disclosed in JP 4403627 as a composition comprising a base resin having an acid labile group other than acetal group, the composition having improved crack resistance. The resin used therein includes units which undergo elimination reaction with an acid and become soluble in alkaline developer, for example, (meth)acrylate units having 1,1-dimethyl-3-oxobutyl and/or 2-benzyl-propan-2-yl as the acid labile group. These acid labile groups have the following characteristics. Now that the solder bump-forming resist composition is formed into a thick film, if a compound resulting from acid elimination has a boiling point of up to 20° C. under atmospheric pressure, it can gasify, remain within the film, and form large bubbles in the film, adversely affecting the pattern profile. For this reason, preference is given to the acid labile group which is acid eliminated to form a compound having a boiling point of at least 20° C. under atmospheric pressure. Allegedly the compounds resulting from acid elimination of the acid labile groups in the units of exemplary base resins have the characteristics.
The acid labile groups mentioned above are tertiary alkyl esters. Under conditions of the step of immersing in a strong acid based solder plating bath, the tertiary alkyl esters do not undergo reaction with the acid in the plating bath, as opposed to the acetal groups. These acid labile groups are characterized by stability and a least likelihood of pattern deformation.
However, the chemically amplified positive resist composition comprising a resin having a tertiary alkyl ester as acid labile group may fail to gain a high resolution in the lithography process of forming a resist pattern because the acid elimination reaction of the acid labile group is inferior to that of acetal groups. This detracts from the removal of resist material to the bottom of a pattern being formed, resulting in a pattern profile being tapered.
When the acid labile group has characteristics as described in JP 4403627, that is, the acid labile group is able to be acid eliminated to form a component having a boiling point of at least 20° C. under atmospheric pressure, a possibility that the compound resulting from acid elimination in the resist pattern undergoes secondary addition reaction with the polymer or other resist components or reverse reaction to generate the acid labile group again cannot be completely removed. For this reason, the resist material film which is to become alkali soluble after exposure and development is extremely reduced in alkali solubility, failing to gain a high resolution. If the compound resulting from acid elimination has a high boiling point or a large or bulky molecule, the secondary addition reaction or reverse reaction as pointed out above can take place significantly. The resist material becomes insolubilized and left where spaces must be formed via positive/negative reversal, that is, as positive tone. Inversely a phenomenon to form a pattern there or a problem to induce scum formation can take place. Since the film formed of the solder bump-forming resist material is as thick as several tens of μm to about 100 μm, there is a tendency that the secondary addition reaction or reverse reaction as pointed out above takes place.
Photoacid generators generally used in chemically amplified positive resist materials include organic acid generators of onium salt type and of non-onium salt type such as oxime sulfonate derivatives. When a chemically amplified positive resist film is patterned on a metal substrate such as copper substrate which is to be subjected to plating to form solder bumps, the acid generated by the acid generator may be deactivated by the influence from the substrate, or the acid generator itself may exhibit a quasi-decomposition behavior by the influence from the substrate. In this way, the acid generation function can be lost. The portion of the pattern which is located near the substrate is observed to be degraded, which is known as a footing profile or undercut profile that the pattern portion appears to bite into the substrate. The footing profile can be transferred to solder bumps near the substrate as cutouts, with a possibility that the pattern collapses. The undercut profile also degrades the profile of subsequently formed solder bumps. That is, the profile of the resist pattern is transferred to solder bumps, and footing of solder bumps is thus observed. Because of the footing, the size of solder bumps near the substrate deviates from the target value.
For the chemically amplified positive resist material used in the formation of solder bumps via a plating step, the selection of acid labile group is critical. It is critical to select an acid labile group which is stable against strong acid in the plating bath, which affords high reactivity with the acid generated by the acid generator so that acid elimination reaction may smoothly run, and thus displays a high resolution, and which does not detract from the high resolution performance since the elimination reaction does not entail secondary addition reaction or reverse reaction, when the relevant resist composition is subjected to the lithography pattern forming process. In addition, it is also important to seek for a resist composition from which a resist pattern can be formed on a metal substrate, typically copper substrate for solder bump formation, without any degradation like footing and undercut. It is urgently demanded to seek for an acid generator or to construct a resist material which avoids any degradation of the pattern profile by generation of bubbles or the like after acid elimination when subjected to the lithography pattern forming process.